HYBRIDS 117 



horse is an addition to the recapitulation and not a re- 

 gression. Reversed selection then is a prolongation of the 

 life-history, while reversion is a shortening of it. 



The results of crosses between different natural races and 

 species are very interesting when considered in connection 

 with the theory of recapitulation. Sometimes the progeny 

 are fertile, but often they are sterile. Thus the mules 

 produced from a cross between a donkey and a pony are 

 almost always sterile. Crosses between domestic European 

 cattle and bison have been found fertile. Among plants, 

 crosses between different species are frequently fertile. An 

 attempt has been made to explain these facts by supposing 

 that the number of chromosomes is different in the two races 

 which when crossed produce sterile offspring, while it is the 

 same in those cases where the progeny are fertile. We have 

 seen that the chromosomes cannot possibly be the bearers 

 of racial characters, so this interpretation does not seem 

 probable. There is some evidence which suggests that 

 difference in the number of chromosomes in the two parents 

 does not necessarily render the progeny sterile, 1 although 

 the evidence is not direct or conclusive. 



One of the most interesting points about the crosses 

 of natural races is, that generally a blend between the 

 characters of the two parents is found in the offspring, 

 whereas in a cross between different artificially produced 

 races the offspring generally stop short, and do not develop 

 the recently produced characters. For instance a cross 

 between the common English trout (Salmo fario) and 

 American brook trout (Salvelimus fontinalis) blends the 

 characters of the parents. Sometimes a character resembles 

 that of one parent more than that of the other. For instance 

 the shape may be more like fontinalis than/ario, but the 

 reverse also occurs. The progeny of the cross are sterile. 2 



1 Rosenberg, 0. , "Das Verhalten der Chromosomen in einer hybriden Pflanze," 

 Berichte der Deutschcn Botanischcn Gescllschaft, 1903. 



2 Information with regard to this cross was kindly given to me by Mr. F. G. 

 Richmond, The Surrey Trout Farm, Shottermill, Haslemere ; Mr. J. B. Fielding^ 



